The invention relates to a method of detecting blind a mode used to encode digital data and can be applied to the transmission of encoded speech signals on a wireless-mobile channel.
With applications which transmit speech on a wireless-mobile channel at low rate, such as transmission of this type of signal at 13 bits/s from the GSM speech encoder, it is currently necessary to protect the transmitted data, on the one hand because the transmission channel is of mediocre quality and on the other because transmission errors have a subjective effect on the audio digital signal which very rapidly becomes unpleasant for the listener.
Since the line rate is fixed at 22.8 kbits/s for full rate GSM transmission or at 11.4 kbits/s for half rate GSM transmission, the fact that not all the bits of a data frame emitted by the voice encoder are of equal subjective importance is used as a means of protecting only part of this frame by using an encoding system of the convolutional coding type. At present, the 260 bits of a full-rate speech frame are split into two classes of protection: 182 bits of this frame are protected by convolutional encoding with a xc2xd output and the remaining 78 bits are transmitted without protection, the latter bits being regarded as the least sensitive. This system of applying a split into two categories of protection is rigid and does not depend on the quality of the wireless transmission channel.
The objective of the present invention is to implement a method of detecting blind, on reception, the mode by which digital data was encoded, so as to allow on emission, i.e. at the level of the encoding process, the system of sharing the overall transmission rate, which is assumed to be constant, between voice encoder and channel coding to be adapted so that if the transmission channel is good, the rate of the speech encoder is increased at the level of the voice coder and that of the channel encoding is reduced, given that the introduction of high redundancy by convolutional encoding is not indispensable, in order to improve the audio frequency quality of the transmitted signal, whereas, if the channel is poor, a switch is made to an encoding mode in which the intrinsic quality of the coding at the level of the voice coder is reduced to the benefit of the channel coding. With an operating mode of this type, it will be conceivable to maintain the continuity of the radio frequency link albeit with an audio frequency of an inferior quality.
Accordingly, and in accordance with one of the object of the invention, the aim is to provide a method that will allow the mode used to encode digital data to be detected blind at the level of any receiver and distinguished from a given number of coding modes of the convolutional type, it being possible for each coding mode to be characterised and distinguished on the basis of the number of bits coded or not coded in each frame making up the audio digital signal.
Another object of the present invention is to provide a method of detecting, blind, the mode used to encode digital data which obviates the need to transmit the coding mode used to the receiver, by means of a certain number of bits, this type of operating mode using a certain number of bits which can not be used to transmit data in the successive frames, which must also be subjected to a very robust form of convolutional coding to protect against transmission errors, without which the risk that decoding would be attempted on the basis of incorrect information relating to the coding mode used would be unacceptable.
The method proposed by the invention of detecting, blind, a mode used to encode digital data transmitted in frames, in which each current frame comprises n bits and consists of a first given number of bits coded by a coding system and a second given number of non-coded bits, the number of coded bits and the number of non-coded bits which define the coding mode being chosen from a finite set of possibilities, is noteworthy in that it consists in computing a soft or flexible syndrome value representative of the transmission error probability for the bits in question over a sequence of bits for at least one frame transmitted, computing, on the basis of parameters of the mean and variance type relating to the distribution of these soft syndrome values, a value of probability, depending on the number of coded xcex1 or non-coded bits xcex2=nxe2x88x92xcex1, that xcex1 bits and soft syndrome values and xcex2 bits and soft syndrome values will correspond to a coded part or non-coded part respectively, these numbers xcex1 and xcex2 being representative of the possible coding mode used, maximising this probability value as a function of one of the numbers representing the possible coding mode used, which will allow the actual coding mode used to transmit this digital data to be detected for the maximum value of this probability value.
The method proposed by the invention can be applied to the transmission of digital data by packets on a channel having characteristics which vary over time, such as radio, Hertzian or satellite channels.